I. Field of the Invention
The invention relates to wireless communication systems. More particularly, the invention relates to techniques of interfacing data terminal equipment units to wireless data networks.
II. Description of the Related Art
Large networks for interconnecting computers have been readily available and yet constantly evolving since the mid-1980""s. The largest and most commonly known digital data network is the Internet. In general, networks provide for the communication of digital data between two data terminals. In addition to the Internet, many organizations have private networks to which access is limited to a select number of users. For example, a corporation may have an internal data network which interconnects its computers, servers, dumb terminals, printers, inventories and test equipment using a wired Ethernet topology.
Typical networks operate based on the transfer of groups of bits, called frames. Thus, an entire file of data, such as an e-mail message, is segmented into a series of frames for transmission over the network. The actual data represented in each frame is attached to a series of headers associated with a set of protocol layers. Each protocol layer is devoted to handling one or more of the issues involved with the transportation of data between terminals.
Most digital networks are comprised of many nodes. On its journey through the network a frame may pass through a series of network nodes. The nodes may be repeaters, bridges, routers, switches or gateways depending, generally, on the highest protocol layer which is examined by the node and whether the node transforms the data from one transport protocol to another.
The nodes maybe connected using a variety of different physical media, referred to as links. With the advent of wireless communication techniques, some data networks now include nodes which are connected by a wireless medium. Through the use of such a system, rather than being tethered to a wired connection to a physical data port the user may transport his terminal to a remote location and still have access to the network.
FIG. 1 is an exemplary embodiment of a terrestrial wireless communication system 10. FIG. 1 shows three remote units 12, 13, and 15 and two base stations 14. In reality wireless communication systems may have many more remote units and base stations. In FIG. 1, the remote unit 12 is shown as a mobile telephone unit installed in a car. FIG. 1 also shows the fixed location remote unit 15 in a wireless local loop system and the portable computer remote unit 13 in a standard cellular system. In the most general embodiment, the remote units maybe any type of communication unit For example, the remote units may be hand-held personal communication system (PCS) units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment. FIG. 1 shows a forward link 18 from the base stations 14 to the remote units 12, 13 and 15 and reverse link 19 from the remote units 12, 13 and 15 to the base stations 14.
Several commercial systems exist which provide true mobility to the user for data and voice services. The system illustrated in FIG. 1 may use code division multiple access (CDMA), time division multiple access (TDMA), a combination of frequency hopping and TDMA (such as Global System for Mobile Communication (GSM)) or other modulation and access techniques. In the past standards using each of these communication techniques have been initially developed to provide voice services. The remote units using existing voice standards have been adapted to provide data servicesxe2x80x94for example, to act as nodes within a digital internetwork.
However, higher performance wireless data networks may be developed for data-only applications. Without the need for constraints imposed by voice operation, a data-only network can be designed which provides much higher rates than that achievable by the adaptation of the wireless voice systems.
FIG. 2 illustrates a prior art system which provides wireless digital communication for terminal equipment unit 30. Generally, the terminal equipment unit 30 may be any type of terminal which produces digital information. For example, the terminal equipment unit 30 may be a personal notebook computer, a printer, test equipment, a server, a dumb terminal or a variety of other equipment. Digital data produced by the terminal equipment unit 30 is passed to a wireless modern 32 via a standard digital link. For example, the terminal equipment unit 30 maybe connected to the wireless modem 32 via a bi-directional RS-232 bus. The wireless modem 32 provides an interface to a wireless link 34 and network infrastructure 36. When communications are estblished between the terminal equipment unit 30 and other equipment connected to the network infrastructure 36, the wireless modem 32 and the terminal equipment unit 30 exchange digital data over the RS-232 bus. The RS-232 bus is an industry standard serial link physical layer over which data formatted in a variety of higher layer protocols may pass.
As noted above, with the advent of data-only wireless networks, the speed at which data may travel across the wireless link has increased significantly. In fact, today the data rate which maybe achieved over the wireless link exceeds the maximum data rate which may pass over an RS-232 link. In order to reap the benefits of increased data speeds over the wireless link the wireless modem must exchange data with the terminal equipment unit at a rate which is similar to the highest data rate available over the wireless link. Thus, it becomes apparent that the RS-232 link, as used in prior art configurations, becomes a limiting factor which prevents the terminal equipment unit 30 from taking advantage of the increased data rates available on the wireless link 34.
One important criterion when designing a wireless modem is that the interface with the terminal equipment unit be in accordance with an accepted industry standard. Aside from RS-232, another such industry standard is Ethernet. Ethernet was developed for wired applications and is not particularly suited for a wireless environment. For example, Ethernet is a broadcast system. In other words, all stations on an Ethernet network receive all messages regardless of whether the station represents the intended destination. Each station must examine the received frames to determine if the station is the destination. If the station is the intended destination, the frame is passed to a higher protocol layer for appropriate processing at the station. If the current station is not the proper destination, the station simply discards the frame.
Each station in an Ethernet system is assigned a hardware address. The hardware address is contained on the Ethernet interface card and is permanent and unique to the Ethernet interface card hardware. Groups of Ethernet stations which time multiplex their signaling on a common Ethernet connection are called a subnet. When a station on an Ethernet subnet sends a message to a station outside of its Ethernet subnet, it sends it to a gateway associated with the Ethernet subnet. Each Ethernet frame begins with a preamble. Following the preamble, the frame comprises the hardware address of the destination station and the hardware address of the source station. Within the Ethernet frame, the source address is always a unicast address meaning that the source always indicates the address of the terminal equipment unit which is sending the message. The destination address maybe unicast (to single node), multicast (to group of nodes), or broadcast (to all nodes on the Ethernet subnet) address.
Because of the finite availability of spectral resources, the capacity of the wireless link is a very precious resource. Broadcasting a single message to a large number of stations consumes significant resources. For this reason, it is not practical to operate a wireless link using Ethernet-type broadcast messaging.
However, as stated above, it is important to use standard terminal equipment unit interface mechanisms when introducing a wireless link into a digital data network. Therefore, a need exists in the industry to provide an interface between standard terminal equipment units and a high speed wireless link.
A wireless modern is used to connect a wireline broadcast medium terminal equipment unit such as an Ethernet unit to a network unit over a wireless link. In order to avoid the transmission of broadcast messages from the network unit over the wireless link, a local server is incorporated within the wireless modem. When the terminal equipment unit sends a request for an address to the network unit the wireless modem intercepts the message. The local server assigns an IP address to the terminal equipment unit and responds to the terminal equipment unit In this way, the terminal equipment unit need not be modified in any way.
The wireless modem receives packets from the terminal equipment unit over the wireline broadcast medium from the terminal equipment unit and forwards it over the wireless link to the network unit using a conventional wireless protocol. In addition, the wireless modem routes packets received over the wireless link addressed to the terminal equipment unit to the wireline broadcast medium.